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The anterior aspect of a human eye generally includes a clear, dome-shaped cornea that covers the anterior chamber and iris. Light passes through the cornea, through the clear fluid that fills the anterior chamber, through an opening in the iris and then through the eye's lens. The cornea is devoid of blood vessels, except at its margins, but it does contain many nerves. The cornea receives nutrients and oxygen from tears which bathe its anterior surface and aqueous humour which contacts the posterior side of the cornea.
The cornea helps to focus light as it enters the eye. The curvature of the cornea provides its focusing power. Light entering the eye is partially refracted by the cornea before reaching the lens. Also, the cornea serves as a protective cover to prevent foreign matter from injuring the pupil, the iris or the inside of the eye.
The cornea has an outer (anterior) epithelial layer, an inner (posterior) endothelium and a relatively thick stroma positioned between the epithelial layer and endothelium. A thin, smooth membrane, known as Bowman's Layer, lies between the epithelial layer and the anterior surface of the stroma. Another thin membrane, known as Descemet's Layer, lies between the posterior surface of the stroma and the endothelium. The stroma, as well as Bowman's Layer, contains strong collagen fibers which define the shape of the cornea. The collagen fibers within the stroma are arranged in a regular, geometric fashion which provides the needed transparency.
A number of pathological disorders may cause the shape of the cornea to change adversely. Generally, Corneal Ectasia is caused by biomechanical weakening or destabilization of the cornea. Corneal Ectasia sometimes occurs as a complication of refractive surgery such as LASIK. In one type of Corneal Ectasia, known as Keratoconus, the cornea thins and becomes abnormally conical in shape. Keratoconus is relatively common, affecting about one person in a thousand. At present, Keratoconus and Corneal Ectasias resulting from refractive surgery are common indications for corneal transplantation. However, corneal transplantation is expensive, requires substantial recovery time, can utilize scarce donor tissues and has inherent risks of post-surgical complications. Thus, any treatment that can delay or prevent the need for corneal transplantation in these patients may be of substantial benefit.
Orthokeratology is a process that uses specially designed rigid contact lenses to temporarily reshape the contour of the cornea to correct refractive errors resulting from routine disorders such myopia or other pathologies such as Corneal Ectasia or Keratoconus. Normally, the corrective orthokeratology lenses are worn only at night. In some cases, a series of orthokeratology lenses having progressively greater curvature are used over a period of days or weeks to achieve the needed degree of corneal reshaping. After the desired reshaping of the cornea has been attained, the cornea tends to revert back to its original shape unless measures are taken to maintain the orthokeratologically-corrected corneal shape.
One measure that is sometimes taken to maintain the corrected corneal shape is to periodically insert and wear a specifically shaped orthokeratology lens (e.g., a “retainer”) to maintain the corrected corneal shape. Another approach that has been described is “fixing” the cornea in its corrected shape by crosslinking of corneal collagen fibers. Crosslinking of corneal collagen fibers without orthokeratology has also been used and reported as a means for deterring progression of corneal disorders such as Corneal Ectasia or Keratoconus. Generally, crosslinking of corneal collagen has heretofore been effected by administering ultraviolet A light (UVA) combined with riboflavin (Vitamin B2). Typically, in this procedure, anesthesia drops are administered to the eye and the epithelial layer is removed. Riboflavin drops are then administered. The riboflavin acts both to enhance the crosslinking effect of the UVA and, also, to absorb a substantial amount of the UVA thereby preventing it from damaging the retina or other deeper structures of the eye. After the riboflavin has been administered, the patient must look into an extracorporeally-positioned ultraviolet light for a period of time (e.g. 30 minutes). At the conclusion of this procedure, a corneal bandage in the nature of a soft contact lens is applied to the anterior surface of the cornea from which the epithelium has been removed. This corneal bandage is typically left in place for a number of days and must then be removed. Antibiotic and anti-inflammatory drops are typically used for about two weeks after the procedure.
Also, U.S. Patent Publication No. 2001/016,731 (Devore et al.) describes an orthokeratology method that includes the steps of softening of the cornea with a softening agent, applying a mold (e.g., a shaping contact lens) to reshape the cornea to a desired anterior curvature, and rapidly restabilizing or “fixing” the corneal tissues so that the cornea retains its new configuration. A chemical softening agent, such as glutaric anhydride is applied to the cornea to soften the cornea, after which a specially designed mold of predetermined curvature and configuration is applied to the cornea. Slight downward pressure is applied to the mold for a predetermined period of time to re-shape the cornea. The mold is maintained in position while a stabilizing agent, such as a UV light source, is positioned above the mold (i.e. not in direct contact with the patient's eye. The UV light, is applied to the cornea for a predetermined time to “restabilize” the corneal tissue so that the cornea retains its shape upon removal of the mold. The stabilization process can also be used for patients having already undergone traditional orthokeratology to eliminate the need to continue wearing a retainer to maintain the shape of the cornea.
There remains a need in the art for the development of new devices and methods for crosslinking corneal collagen in ways that are safer, easier and potentially less costly.